20. 有效的括号
Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.
An input string is valid if:
Open brackets must be closed by the same type of brackets.
Open brackets must be closed in the correct order.
Every close bracket has a corresponding open bracket of the same type.
#1 自己看到题目的第一想法
新建一个array，用pop和push即可解决。就是如果array是空的时候需要单独的处理一下。
#2 看完代码随想录之后的想法
由于栈结构的特殊性，非常适合做对称匹配类的题目。
1047. 删除字符串中的所有相邻重复项
You are given a string s consisting of lowercase English letters. A duplicate removal consists of choosing two adjacent and equal letters and removing them.
We repeatedly make duplicate removals on s until we no longer can.
Return the final string after all such duplicate removals have been made. It can be proven that the answer is unique.
#1 自己看到题目的第一想法
同上题
150. 逆波兰表达式求值
#1 自己看到题目的第一想法
同上题
#2 收获
这几道题都非常的类似，都是新建一个stack，然后比较当前的元素和stack.pop()，难度不难，写的时候注意一些细节就好
